Laser excited ultraviolet fluorescence of tryptophan peptides was studied as a model for protein fluorescence, which arises from tryptophan residues. From the heterogeneity of the fluorescence parameters, it could be deduced that these peptides assumed different conformations in solution. Some conformations were non-fluorescent due to static self-quenching, a phenomenon which was demonstrated for the first time. Such quenching probably occurs in proteins, explaining certain puzzling features of their fluorescence. The detailed physical processes occurring after photoexcitation of indole derivatives such as tryptophan was studied by fluorescence and n.m.r. analysis of products formed by irradiation with ultraviolet irradiation in heavy water (D20). Proton exchange occurred with some compounds but not others. Proton transfer thus is not necessary for quenching of indolyl fluorescence, but may occur secondarily after electron delocalization. Energy transfer between protein tryptophan residues and attached dyes was studied using the technique of global analysis of fluorescence decay. Combined with spectral data, this method was able to determine the distance between energy donor and acceptor without prior knowledge of the degree of binding.